Revert "nixos/dnscrypt-proxy: remove"

This reverts commit 5dc2853981.
The project has a new maintainer.

nixos/tests/dnscrypt-proxy.nix

@@ -0,0 +1,32 @@
+import ./make-test.nix ({ pkgs, ... }: {
+  name = "dnscrypt-proxy";
+  meta = with pkgs.stdenv.lib.maintainers; {
+    maintainers = [ joachifm ];
+  };
+
+  nodes = {
+    # A client running the recommended setup: DNSCrypt proxy as a forwarder
+    # for a caching DNS client.
+    client =
+    { config, pkgs, ... }:
+    let localProxyPort = 43; in
+    {
+      security.apparmor.enable = true;
+
+      services.dnscrypt-proxy.enable = true;
+      services.dnscrypt-proxy.localPort = localProxyPort;
+      services.dnscrypt-proxy.extraArgs = [ "-X libdcplugin_example.so" ];
+
+      services.dnsmasq.enable = true;
+      services.dnsmasq.servers = [ "127.0.0.1#${toString localProxyPort}" ];
+    };
+  };
+
+  testScript = ''
+    $client->waitForUnit("dnsmasq");
+
+    # The daemon is socket activated; sending a single ping should activate it.
+    $client->execute("${pkgs.iputils}/bin/ping -c1 example.com");
+    $client->succeed("systemctl is-active dnscrypt-proxy");
+  '';
+})